eli5: How exactly are recharable batteries recharged?


I know that in a normal battery once the electrons have made their way from the anode to the cathode the battery dies, but rechargeable batteries are able to sort of reset the battery once it’s been put on charge.

Every time I watch a YouTube video on rechargeable batteries all they say is “We do this by forcing the electrons back to the anode” without explaining exactly how that works and its driving me up the goddamn wall.

In: 15

A battery has a specific voltage. To recharge a rechargeable battery, you put a slightly higher voltage on the electrodes. The battery behaves sort of like a resistor while the electrochemical reactions are going in reverse.

You just put a voltage slightly higher than the battery’s own on the terminals. It will overcome the electromotive force of the battery and force the charges to move backwards through it. The chemical process is reversible so the charges moving backwards through the cell cause the chemicals in the battery to migrate back to the starting position.

You need to do it very carefully though, if you do it too fast you can cause gases to come out of the electrolyte solution or excessive heating and blow the battery. For li-ion you need a complex charger circuit that goes through a phase of being current limited (variable voltage) to finishing off with a constant voltage phase with a cell voltage or time cutoff, or you’re going to experience the fun of a lithium battery fire.

Electrical potential energy is stored in the chemicals in the battery waiting to push the electrons around a circuit as soon as the battery is connected and the electrons have somewhere to go.

When the battery is connected, a chemical reaction releases the electrons so they can flow. Once the electrons have flowed through a circuit or device and the voltage that pushed them is no longer there, the battery is dead.

The chemical reaction in a rechargeable battery can be reversed so that the electrons released can be restored to their original place (or very nearly – they don’t last forever) by adding the energy required to push them back to where they were. This is done by applying a reversed voltage to the battery and putting in energy from an external power supply such as a solar panel or mains plug using a charger.

By supplying a voltage from a power source to push the electrons back in the other direction, the chemical reaction is reversed and some energy to move the electrons is put back into the battery and stored in the chemicals, and can be released by reusing the battery.

It’s a little like the way your toilet flushes: potential energy (in that case gravitational rather than electrical) from the water stored in the toilet tank is used to move the waste out of the toilet – so we add more energy by refilling the tank from the water main once the flush is done so we add more potential energy to the system and it’s “recharged” and ready to use again.

For starters to recharge anything, the voltage of the charger or charging battery (jumpstarting) has to be of higher voltage than the battery being charged, otherwise the current goes in the opposite direction.

You can think of voltage as a force. In order to change course, you have to have a higher force.

The ability to recharge comes from the chemistry. Basically the chemical process is reversible. Depending on the chemistry of the battery when fully charged will be some chemical make-up, and then when it discharges becomes another chemical. When you apply the charger you reverse it back to the original chemical make up. The process isn’t perfect and over time it loses its ability to reverse the chemicals. Which is why they lose performance over time (assuming no physical defect).

Like you’re five: the battery is full of acid. The acid pulls electrons from the negative side and puts them on the positive side. This outputs a Voltage. To charge it, we do the reverse. We input a Higher voltage to the battery which forces the electrons to go from the positive side to the negative side.

Bonus: you technically can recharge dead alkaline batteries. But they weren’t designed for that so they have problems with leaking and degradation